Gravity: We might have been getting it wrong
phys.org
phys.org
Par for the course for phys.org.
As someone very not versed in physics, the paper is actually... really readable so far and short (I'm almost done). At the very least, there is more information than anything mentioned in the links/summaries I could find.
1810.05337 is a summary of results for PRL
Both are very good
A better article with actual content:
[...] the holographic principle allows physicists to study gravitational systems by projecting them on a boundary
[...] proved that symmetry is not possible in a gravitational theory if it obeys the holographic principle.
[...] meaning that symmetry would not be possible in quantum gravity.I strongly recommend anyone interested in physics check out the channel
> TLDR:
> the holographic principle allows physicists to study gravitational systems by projecting them on a boundary
> proved that symmetry is not possible in a gravitational theory if it obeys the holographic principle.
> meaning that symmetry would not be possible in quantum gravity.
Is that true?
The AdS universe is basically a mathematical toy for physicists to play with. It doesn't correspond with the physical structure of the real universe. There's a hope that it might correspond to some kind of mathematical meta-structure that defines physical laws.
This article suggests that's very unlikely.
Personally I wouldn't be surprised if AdS/CFT turned out to be a dead end - probably like supersymmetry - and the real structure of quantum gravity turned out to be completely different and unexpected, and not a Conformal Field Theory at all.
But IANAP, so I may well be wrong about that.
Particle physics is currently defined according to the Standard Model. Dark matter was proposed because it plugs many of the gaps of universal expansion in contrast to the Standard Model, but not all the gaps. It would take far greater effort to invalidate the Standard Model and all of particular physics built upon it. There is also the problem of not having any other model to replace the current one. https://en.m.wikipedia.org/wiki/Standard_Model
The expansion of the universe is observed as red shifts and these observations are numerous and largely uniform. Dark matter, on the other hand, has not been directly observed.
* https://arstechnica.com/features/2014/07/dark-matter-makes-u...
* https://arstechnica.com/science/2019/11/dark-matter-link-to-...
* https://arstechnica.com/science/2019/02/more-bad-news-for-co...
* https://arstechnica.com/science/2019/04/one-night-of-telesco...
* https://arstechnica.com/science/2017/02/a-history-of-dark-ma...
* https://arstechnica.com/science/2016/07/dark-matter-still-mi...
https://en.m.wikipedia.org/wiki/Dark_matter
Even Wikipedia says dark matter is a stop gap of gravitational models. The very next sentence, first paragraph, says experts believe dark matter exists for that reason.
> Without introducing a new form of energy, there was no way to explain how an accelerating universe could be measured.
https://en.m.wikipedia.org/wiki/Dark_energy
Dark energy is an unobserved stop gap as well. Dark energy was introduced because dark matter was not enough of a stop gap to qualify the acceleration of the expansion.
The reality is that there is just as much theoretical qualifiers for the presence of dark matter as there are unexplained phenomena in the standard model that needs dark matter as a qualifier.
The best possible answer to this nonsense is that the standard model is wrong because we don’t have the evidence to form a better model. All our evidence is limited to observations from Earth surface and low orbit. Hopefully the James Webb telescope will provide better evidence.
This statement is way, way too strong. We have a theory of gravity, General Relativity, which has had its predictions confirmed by countless experiments, in some cases to thirteen or fourteen decimal places. That is not "getting it wrong".
Many physicists believe that GR, as well confirmed as it is, can't be the final fundamental theory of gravity because it's not a quantum theory. But nobody has yet found a quantum theory of gravity that (a) is consistent with the rest of quantum mechanics, and (b) makes experimental predictions that have been confirmed. Unless and until both (a) and (b) are done, the claim that we are "getting it wrong", even in the restricted sense of not having a final fundamental theory of gravity, because GR isn't a quantum theory is just speculation and might itself be wrong. It should certainly not be stated with the confidence you are stating it.
See here for instance a popsci take on the problem: https://www.scientificamerican.com/article/is-gravity-quantu...
Everybody expects a quantum theory of gravitation, but if we were drawing it from scratch no one would make it that weak. It is a difficult problem, people work in string theory because you can get Einstein's equations from strings and the alternatives look indeed worse. I don't think that means GR is a wrong take on gravitation, it's more like an amazing one to begin with.
Most likely because they'd rather downvote rather than challenge your statement. It's a worrying trend.
Then why does GR make more accurate predictions?
So there is no global "right" or "wrong", just different measures of usefulness depending the problem constraints.
When measuring signal (ie radio, light, gravity) from slow speed perspectives, newtonian physics is not as accurate - https://futurism.com/newtonian-physics-vs-special-realtivity - Eg we actually need to account for it between gps sattelites and we do so from the perspective of GR as well as the recent evidence of gravity propogation being restricted to localization (ie roughly the speed of light)
No, because "less accurate but in practical terms the difference can often be ignored" is not the same as "the same accuracy".
You are wrong, so there is.
GR is always more accurate, but the increased accuracy is not always needed; at low speeds relative to the speed of light the difference is often too small to matter in practical terms.
i think we're on a right way in general :
QM -> GR : https://en.wikipedia.org/wiki/Quantum_gravity#Quantum_mechan...
" In particular, contrary to the popular claim that quantum mechanics and general relativity are fundamentally incompatible, one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles (called gravitons)"
GR -> QM : We did observed gravitational waves of GR. A force field with waves mediating that force interactions - that requires just a bit of math to "quantize" it. It is a bit more complicated when classic EM -> QM because of added complication of non-stationary spacetime and just vastly different scales of spacetime/energy to run the experiments to establish/verify the gravitons parameters.
For the "true deep" nature of graviton I subscribe to the view that - given gravitational force not being the "real" force and the gravitational waves being really a ripples of spacetime - graviton is more like phonon, ie. quasi-particle from the point of view of QM/SR theories.
While this is technically true, it doesn't help much, since the quantum theory described is not considered a viable candidate for a fundamental theory of gravity for a number of reasons.
> We did observed gravitational waves of GR. A force field with waves mediating that force interactions
We have observed classical gravitational waves. We are many, many orders of magnitude away from being able to measure any possible quantum properties of those waves.
> that requires just a bit of math to "quantize" it
Math (more than "a bit") can quantize it in theory. But theory is not enough. We would actually have to observe the quantum properties experimentally to confirm the theory.
While not directly quantum properties of gravitational waves yet, we've already observed quantization of gravitational potential (and that naturally suggests "gravitational quantas" at least by similar machinery as with EM->QM) - at small scales/energies when the spacetime curvature can be not paid attention to, the gravity potential of an ultra-cold neutron quantizes just nice using the Schrodinger equation in full agreement with experimental observation - https://www.physi.uni-heidelberg.de/Publications/dipl_krantz... . In my view, giving the amount of clarity in gravitation as well as in QM that experiment produced, it is worth of a Nobel.
No, we haven't. We've observed that, under appropriate conditions, gravitational potential has to be included in the potential term in the Hamiltonian. But the potential term in the Hamiltonian is not quantized; it's not part of the quantum state and it does not exhibit quantum properties. The only things in the experiment that exhibit quantum properties are the neutrons themselves.
It is like saying that discrete orbits of electron say nothing about quantization of associated EM potential.
>The only things in the experiment that exhibit quantum properties are the neutrons themselves.
And those properties are position and momentum.
They specifically chosen neutrons to avoid effect of other forces and the paper is pretty clear that position and motion of neutrons were primarily affected by gravitational field, and as result the neutron position got quantized :
"... we conclude that the measurement manifests strong evidence for quantisation of motion in the gravitational field as is expected from quantum mechanics
...
the quantum theory we derived neglecting gravity is unable to reproduce the shape of the neutron height distribution even on the largest scale.
..."
Do you think the height of observed "steps" in neutron position would be the same or different if the experiment were repeated in different gravity, say Moon?
That's right, they don't. There is no such thing as "quantization of EM potential". The electromagnetic field is quantized, if you're using quantum field theory, but in QFT there is no "EM potential". In non-relativistic QM, the EM potential term in the Hamiltonian is not quantized and does not exhibit quantum properties. Even in models which add relativistic corrections, such as the models used in the late 1940s to predict the Lamb shift, the "quantization of the EM field" only applies to the "quantum electromagnetic field" external to the atom; it does not apply to the EM potential due to the nucleus.
> the paper is pretty clear that position and motion of neutrons were primarily affected by gravitational field, and as result the neutron position got quantized
No, the neutron positions were not quantized "as a result" of the gravitational field. They are already quantized in the absence of a gravitational field, as shown by numerous previous experiments. The only thing this experiment shows, as I've already said, is that under appropriate conditions, you have to include the gravitational potential in the potential term in the Hamiltonian.
Obviously since the gravitational potential would be different, the neutron position "steps" would be different. That still doesn't change the fact that the experiment is not showing "quantized gravitational potential". It's only showing that "quantized neutron position", which is already well established by other experiments, is affected by the gravitational potential in the same way as it would be by any other classical potential in the Hamiltonian.
rounding error in The Grand Simulation.
I haven't really seen science work that way, since it isn't all that often we base our expectations solely on things we can completely account for. Science necessitates a certain amount of calculated projection. Theories are adjusted as new details are discovered, and only fools entertain themselves by resisting the bigger leaps needed to make actual discoveries.
The obvious parallel was https://en.wikipedia.org/wiki/Pioneer_anomaly which was finally solved. But who know at this point?
Personally I think GR has to be "wrong", since it conflicts with QM, and if the 20th century showed us anything it's that QM alway wins.
It was my (lay) impression that we believe general relativity cannot be the final fundemental theory of gravity because general relativity itself is not consistent with quantum mechanics. Whether general relativity is a quantum theory or not is beside the point; we want to have a theory -- any theory -- that is consistent with itself. Right now we have two separate theories (general relativity / quantum mechanics) that can't be combined because they conflict with each other.
Obviously, that in itself doesn't prove that the problem is in the theory of gravity, but it does prove the problem is either in the theory of gravity or the theory of quantum mechanics, and we have to be getting one of them wrong.
That's the same as "not a quantum theory", which is what I was saying. "Consistent with quantum mechanics" requires "is a quantum theory", according to the many physicists I was referring to.
> Right now we have two separate theories (general relativity / quantum mechanics) that can't be combined because they conflict with each other.
Most physicists believe that they conflict, yes. But not all. Freeman Dyson, for example, has speculated that gravity might just be different and not require a quantum theory the way the other interactions do.
Not true: we could also be getting them both wrong.
When the lay person reads headlines like this, they assume you are talking about Einstein's theory.
https://phys.org/news/2019-05-constraints-symmetries-hologra...
Their work assumes the AdS/CFT correspondence.
"Our new paper provides a rigorous proof of this claim in the context of the AdS/CFT correspondence, where quantum gravity is defined in a mathematically precise way, and we have done so in the most general way, excluding all possible global symmetries from quantum gravity,"
There was also a brief mention of holography in this interview with String Theorist Jim Gates on Lex Fridman's Youtube channel [2]. I can't remember the exact context of how holography is related to strings but I believe he was talking about how it could impact some of the maths behind the multiple dimensions some string theories required.
Every good(!) one knows that we got it wrong. Just a little less wrong than in the previous model. You cannot get it right. It wouldn't be a model but reality as a whole if you got it right. So the only questions are how you can get it less wrong than the last model, and which of the available models is most useful for the problem you are currently tackling. So yes, sometimes you even want to use a more-wrong model because it fits your problem better.
edit: thought about bringing in an example, but honestly it would need too much googling to be precise enough.
Or they are equal in fit, but the more wrong is easier to use.
-K. Vonnegut
We understand enough of the effects gravity to describe most phenomena that are relevant for most other parts of science. From my lay understanding what physicists are missing is how to tie in gravity with all the other theories they believe are true. However the effects, whatever they are, are so small that we can in most cases just ignore them.
This is a scientific result. What are you trying to say?
My incorrect understanding is something like: quantum models imply a uniform layer of “virtual” particle/anti-particle pairs popping in and out of existence. but if the space they’re in is curved like general relativity describes, then the pairs don’t find each and annihilate each other reliably and all your math trends towards infinity instead of towards zero. or something vaguely like that.